Since the discovery of carbon nanotubes (S. Iijima, Nature 354, 56 (1991); T. W. Ebbesen and P. M. Ajayan, Nature 358, 220 (1992)) there have been many applications for such materials. Their size and high aspect ratios leads to possible use as electron emitters for flat panel displays (Q. H. Wang, A. A. Setlur, J. M. Lauerhaas, J. Y. Dai, E. W. Seelig, and R. P. H. Chang, Appl. Phys. Lett. 72, 2912 (1998)) and AFM/STM probes. (H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, Nature 384, 147 (1996).) In addition, the reduced number of defects in nanotubes could make them the ultimate carbon fiber in terms of strength and stiffness. (M. M. J. Treacy, J. M. Gibson, and T. W. Ebbesen, Nature 381, 678 (1996); E. W. Wong, P. E. Sheehan, and C. M. Lieber, Science 277, 1971 (1997).)
Carbon tubules and related nanostructures are typically prepared using standard arc-discharge techniques. Generally, the discharge is in a reaction vessel through which an inert gas flows at a controlled pressure. The potential, either direct or alternating current, is applied between two graphite electrodes in the vessel. As the electrodes are brought closer together, a discharge appears resulting in plasma formation. As the anode is consumed, a carbonaceous deposit forms on the cathode, a deposit that under the proper conditions contains the desired carbon nanotubules.
However, conventional methods of making multi-walled nanotubes via arc discharge do not easily lend themselves to large scale production. (D. T. Colbert, J. Zhang, S. M. McClure, P. Nikolaev, Z. Chen, J. H. Hafner, D. W. Owens, P. G. Kotula, C. B. Carter, J. H. Weaver, A. G. Rinzler, and R. E. Smalley, Science 266, 1218 (1994).)
A variation of this general synthetic procedure is reflected in U.S. Pat. No. 5,482,601, wherein carbon nanotubes are produced by successively repositioning an axially extending a graphite anode relative to a cathode surface, while impressing a direct current voltage therebetween, so that an arc discharge occurs with the simultaneous formation of carbon nanotubes as part of carbonaceous deposits on the various portions of the cathode surface. The deposits are then scraped to collect the nanotubes. The anode must be repositioned respective to the cathode, repeatedly, to provide larger quantities of the desired nanotube product.
Related technology is described in U.S. Pat. No. 5,877,110 whereby carbon fibrils are prepared by contacting a metal catalyst with a carbon-containing gas. The fibrils can be prepared continuously by bringing the reactor to reaction temperature, adding metal catalyst particles, then continuously contacting the catalyst with a carbon-containing gas. Various complexities relating to feed rates, competing side reactions and product purity, among others, tend to detract from the wide-spread applicability and acceptance of this approach.
At low temperatures, namely below 1500xc2x0 C. processes currently used in chemical vapor deposition syntheses of carbon nanotubes require metal catalysts such as iron, nickel or cobalt. This approach necessitates an additional chemical-processing step to remove the metal particle catalysts. In so doing, defects are generated in the carbon nanotubes.
It is possible to make spherical or polyhedral graphitic nanoparticles like those made in the carbon arc by the heat treatment of various carbon materials. (A. Oberlin, Carbon 22, 521 (1984); P. J. F. Harris and S. C. Tsang, Phil. Mag. A 76, 667 (1997); W. A. de Heer and D. Ugarte, Chem. Phys. Lett. 207, 480 (1993); P. J. F. Harris, S. C. Tsang, J. B. Claridge. M. L. H. Green, J. Chem. Soc. Faraday Trans. 90, 2799 (1994).) However, only short ( less than 100 nm) nanotubes have been made by annealing fullerene soot. (W. A. de Heer and D. Ugarte, Chem. Phys. Lett. 207, 480 (1993); P. J. F. Harris, S. C. Tsang, J. B. Claridge. M. L. H. Green, J. Chem. Soc. Faraday Trans. 90, 2799 (1994).). While it is possible to scale up heat treatment methods, significant improvements need to be made in order to produce high quality nanotubes like those produced by the arc method.
Understanding the growth mechanisms for carbon nanostructures is important as a first step towards developing new apparatus and methods for preparation. It is instructive to consider several concepts underlying graphitization in the preparation of carbon nanotubes. Graphitization begins with carbon self-diffusion, leading to order in the ab planes of graphite. (L. E. Jones and P. A. Thrower, Carbon 29, 251 (1991).) Once ordering begins to occur, graphitic regions assemble into a layered structure. In this process, defects are progressively removed from aromatic layers at higher temperatures. (L. E. Jones and P. A. Thrower, Carbon 29, 251 (1991); E. Fitzer, K. Mueller, and W. Schaefer, in Chemistry and Physics of Carbon (Marcel Dekker, Inc., New York, 1971), Vol. 7, p. 237; D. B. Fischbach, in Chemistry and Physics of Carbon (Marcel Dekker, Inc., New York, 1971), Vol. 7, Vol. 1; A. Oberlin, Carbon 22, 521 (1984); R. E. Franklin, Proc. Roy. Soc. A 209, 196 (1951); P. J. F. Harris and S. C. Tsang, Phil. Mag. A 76, 667 (1997).)
When many of the defects initially present in non-graphitizable carbons cannot be removed, extensive formation of three dimensional graphite is prevented. Consequently, graphitization of a carbon material is controlled by the initial structure of the carbon material as well as the processing conditions. (E. Fitzer, K. Mueller, and W. Schaefer, in Chemistry and Physics of Carbon (Marcel Dekker, Inc., New York, 1971), Vol. 7, Vol. 237; D. B. Fischbach, in Chemistry and Physics of Carbon (Marcel Dekker, Inc., New York, 1971), Vol. 7, Vol. 1; A. Oberlin, Carbon 22, 521 (1984); R. E. Franklin, Proc. Roy. Soc. A 209, 196 (1951); P. J. F. Harris and S. C. Tsang, Phil. Mag. A 76, 667 (1997).)
To date, there is a general consensus in the art that carbon vapor in the form of atoms, ions, or small molecules is necessary for multiwalled nanotube growth without metal catalysts. (E. G. Gamaly and T. W. Ebbesen, Phys. Rev. B 52, 2083 (1995); T. Guo, P. Nikolaev, A. G. Rinzler, D. Tomanek, D. T. Colbert, and R. E. Smalley, J. Phys. Chem. 99, 10694 (1995); J. C. Charlier, A. De Vita, X. Blase, and R. Car, Science 275, 646 (1997); Y. K. Kwon, Y. H. Lee, S. G. Kim, P. Jund, D. Tomanek, and R. E. Smalley, Phys. Rev. Lett. 79, 2065 (1997); M. Buongiorno Nardelli, C. Roland, J. Bemhole, Chem. Phys. Lett. 296, 471 (1998).) It has also been proposed that ordered, graphitic precursors are essential for nanotube growth. (J. M. Lauerhaas, J. Y. Dai, A. A. Setlur, and R. P. H. Chang, J. Mater. Res. 12, 1536 (1997).) The complexity of the arc process makes it very difficult to study the formation of these materials or draw any conclusions regarding conditions necessary to maximize optimal growth and/or yield.
Accordingly, there is still a need in the art of manufacturing tubular carbon nanostructures, such as carbon nanotubes, for processes and apparatus therefor, which can provide carbon nanotubes essentially free of carbon overcoat, free of any metal catalysts, and generally free of defects caused by removal of catalytic materials usually present in carbon nanotubes prepared by chemical vapor deposition processes.
There are a considerable number of problems and deficiencies associated with carbon nanostructures of the prior art, with most such shortcomings resulting from the current methods of preparation. There is a demonstrated need for innovative methods of preparation so as to provide such compositions in high yield, at large scale and with the desired mechanical, structural and performance properties.
Accordingly, it is an object of the present invention to provide various methods and/or apparatus, which can be used in the preparation of carbon nanostructures, thereby overcoming the problems, deficiencies and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in that the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of this invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of the present invention.
It can also be an object of the present invention to provide a method of using solid disordered carbon materials in the preparation of multi-walled carbon tubes and/or hollow, tube-like structures of nanometer dimension.
It can also be an object of the present invention to provide a method of using a disordered carbon material, and/or by utilizing techniques and processes analogous to graphitization.
It can also be an object of the present invention to provide a disordered carbon material as a source of pentagonal carbon and/or polyhedral structure enroute to the preparation of various tubular carbon nanostructures.
It can also be an object of the present invention to provide a method of preparing carbon nanotubes under conditions not conducive to carbon sublimation and/or substantially without the use of carbon vapor.
It can also be an object of the present invention to provide a method for preparation of carbon nanostructures of the type described herein, having enhanced and/or superior structural or mechanical properties, including but not limited to overall length and strength.
It can also be an object of one present invention to prepare carbon nanostructures of lengths exceeding those now available through the prior art, including lengths up to and greater than 0.5 xcexcm, through choice of carbon precursor and/or process conditions, none of which would be evident, suggested or taught by the prior art.
It can also be an object of the present invention to provide a method of using temperature, heating rate and/or related process kinetics to control the growth, development and/or structure of carbon nanotubes and/or tubular structures.
It can also be an object of the present invention to provide a method of promoting open-ended growth of carbon nanocompositions, so as to enhance various mechanical or performance properties of the resulting structures.
It can also be an object of the present invention to provide an apparatus to effect one or more of the objects presented herein, as well as to increase the yield and/or production rate of carbon nanotubes.
It can also be an object of the present invention to provide a method and/or apparatus for the batch, continuous or semi-continuous production of carbon nanostructures, of the type described herein, and/or to optimize process conditions enroute to tubules and tube-like structures having enhanced structural, mechanical and/or performance properties.
Other objects, features, benefits and advantages of the present invention will be apparent in this summary, together with the following descriptions and examples, and will be readily apparent to those skilled in the art having knowledge of various synthetic methods, preparation of such carbon compositions and/or apparatus which can be used therewith. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying examples, figures, data and all reasonable inferences to be drawn therefrom.
The present invention, which addresses the needs of the prior art, provides a method of forming tubular carbon nanostructures by heating a disordered carbon precursor in the presence of a gas at a temperature and pressure sufficient to form the tubular carbon nanostructures. The tubular carbon nanostructures made by the methods of the present invention include multiwalled carbon nanotubes having an outside diameter of typically 5 to 40 nm and a length in the range of 50 to 150 [[mm]] micrometers. The method is enhanced by further adding charged particles provided by an electrical discharge between a cathode and an anode. The formation of tubular carbon nanostructures is also enhanced by including a dopant in the disordered carbon precursor. The temperature and pressure at which the methods for forming tubular carbon nanostructures are conducted must be adjusted such that they are below that which would cause sublimation of the disordered carbon precursor. The dopant is preferably amorphous boron which is added in an amount sufficient to increase the length of the tubular carbon nanostructures to greater than 0.5 microns. The disordered precursor useful in the present invention is fullerene soot, bold milled graphite, carbon black or sucrose carbon. The methods provided by the present invention are preferably conducted in the absence of any significant sources of carbon vapor. The methods of the invention further include controlling the arc-furnace temperature and the heating rate of the precursor to form multiwalled carbon nanotubes.
The methods of the present invention also include forming tubular carbon nanostructures by discharging a direct current arc between an anode and cathode, the anode, including a conducting electrode containing a carbon precursor, the discharging occurring in the presence of a gas, at a temperature and pressure such that the carbon precursor is maintained in solid phase and for a period of time sufficient to form tubular carbon nanostructures. When forming the tubular carbon nanostructure is accomplished primarily by heating, the carbon precursor is preferably non-graphitizable carbon. On the other hand, when the method includes heating the carbon precursor with electron charged particles then the preferred carbon precursor is graphitizable carbon. A non-graphitizable carbon includes fullerene soot, carbon black or sucrose carbon. Graphitizable carbon includes PVC. The methods of the present invention can be conducted at the pressure from 50 torr to atmospheric and at a temperature range from about 1500xc2x0 C. to about 3500xc2x0 C.
The present invention also provides an apparatus for forming tubular carbon nanostructures which is preferably an arc-furnace. The apparatus comprises a cathode, an anode opposite the cathode, a source of voltage and current in an amount sufficient to create charged particles and to produce an arc between the anode and cathode, a source of gas to surround the arc, and the source of carbon precursor positioned adjacent the anode and within the arc, such that the arc has a sufficiently high temperature and is maintained at a pressure for a time sufficient to heat the carbon precursor to form carbon nanotubes upon the anode.
The anode may have different geometries. The anode preferably has a recess positioned in the anode to receive the charged particles from the arc. In the recess of the anode the carbon precursor is received. The methods of the invention are conducted in a gaseous environment, wherein the gas is a inert gas or nitrogen. The anode may also be a platform positioned within the arc and optionally including a surface which envelopes the platform to retain the precursor therein. The platform may be further movable through the enveloping structure of the anode. The anode may also be circular platform having radial ribs, and spaces between the ribs for receiving the carbon precursor and also a recess for collecting the carbon nanotubes. The anode can also include a circular platform rotatably attached to the anode such that the platform includes a location for the carbon precursor and a recess for collecting the carbon nanotubes.
The present invention also provides an apparatus for forming tubular carbon nanostructures which includes a resistance furnace having at least one opening adapted to receive a conveyor belt. The furnace further includes a source of carbon precursor, a gas source for adjusting the pressure, a heat source sufficient for the formation of tubular carbon nanostructures at the desired pressure. The conveyor belt is operably connected to the resistance furnace and is utilized to retain the source carbon precursor in the resistance furnace for a period of time sufficient to form the tubular carbon structures. Once they have been formed the conveyor belt takes the carbon nanotubes out of the resistance furnace for delivery to a user.